Various forms of variable dampers have been proposed for use in wheel suspension systems for the purposes of improving the ride quality and achieving a favorable motion stability or driving stability of the vehicle. For instance, when a vehicle makes a turn, the vehicle body undergoes a rolling movement owing to an inertia force (lateral acceleration) resulting from a lateral movement of the vehicle. To control an excessive rolling movement of the vehicle body at such a time, it has been proposed to increase the target damping force of the dampers in dependence on a differential value of the lateral acceleration (roll control). When the vehicle travels over an irregular road surface, the wheels undergo vertical movements, and these movements are transmitted to the vehicle body so that the rider quality of the vehicle may be impaired at such a time. To control the transmission of the vertical movements of the wheels to the vehicle body, and thereby improve the ride quality of the vehicle, it has been proposed to increase the target damping force of each damper in dependence on the sprung mass speed of a vehicle part adjacent to the damper or the wheel associated therewith (skyhook control). See Japanese patent laid open publication (kokai) No. 2006-69527.
According to another proposal based on a skyhook control, the damping force of each damper is varied depending on the input from the road or the condition of the road surface so that the ride quality may be improved. See Japanese patent laid open publication (kokai) No. 05-69716.
However, in conventional skyhook control proposals, because the damper for each wheel is controlled in dependence on the sprung mass speed of a vehicle part associated with the wheel under consideration or, in other words, each wheel suspension system is controlled independently from the conditions of the other wheel suspension systems, certain inconveniences have been known to occur.
For instance, when the vehicle is traveling over a slanted road surface and the wheel loads are thereby unevenly distributed, the behavior of the vehicle such as a vertical movement, rolling movement and pitching movement may not be favorably controlled. More specifically, when a vehicle is traveling a road which is slanted downwardly in a fore-and-aft direction, the wheel load acting on the front wheels increases while the wheel load acting on the rear wheels decreases as compared to the condition where the vehicle is traveling over a horizontal surface. Therefore, the natural frequency of the vertical movement of the front part of the vehicle decreases while the natural frequency of the vertical movement of the rear part of the vehicle increases. As a result, the vertical movements of the front part and rear part of the vehicle are thrown out of synchronism, and this causes a corresponding disagreement between the sprung mass speeds of the front part and rear part of the vehicle. Therefore, as shown in FIG. 49, the skyhook control becomes unable to control the disagreements between the sprung mass speeds of the front part and rear part of the vehicle (as indicated by the difference in the graph of FIG. 49), and this cause an undesirable pitching movement of the vehicle. This creates the need to correct the sprung mass speed of the rear part of the vehicle body to a significant extent with a pitch control (as indicated by the sprung mass speed after correction).
According to the invention disclosed in Japanese patent laid open publication (kokai) No. 2006-69527, the pitch control of the vehicle body is based on the fore-and-aft acceleration of the vehicle. However, because the pitching movement of the vehicle may occur also when the vehicle is traveling over a slanted or sloped road surface even though the vehicle is not accelerating or decelerating, the pitching movement may not be controlled in a desirable manner.
Japanese patent laid open publication (kokai) No. 2006-281876 discloses a technology for determining a target damping force according to the lateral acceleration of the vehicle produced by a turning movement of the vehicle and the lateral acceleration produced by a yawing movement of the vehicle so that the rolling movement of the vehicle may be favorably controlled. In this roll control system, the driving stability of the vehicle can be improved by increasing the gain for the front wheels earlier than increasing the gain for the rear wheels based on the knowledge that the yaw rate is produced earlier on the side of the front wheels that are steered than on the side of the rear wheels that are not steered.
However, according to this prior art, when the vehicle makes a turn, and the damping force for the front side is increased earlier than that for the rear side, because the vertical wheel load of the inner front wheel diminishes while the damping force thereof increases, the overall cornering force of the front side is reduced as compared to that of the rear wheel side so that the turning response or the steer feel of the vehicle may be somewhat impaired. In other cases, the driving stability may be desired to be improved while the turning response is controlled. In short, there is a demand for a technology that enables a desired turning response of a vehicle to be achieved at will.
When the damping force is controlled individually for each wheel, as there is no consideration for the influences from the conditions of the remaining wheels, it was noted that the dampers may interfere with one another, and this could cause the attitude control of the vehicle to be impaired at least in transient situations. Japanese patent laid open publication (kokai) No. 2002-127727 discloses a wheel suspension system using variable hydraulic dampers in which oil chambers of different dampers are communicated with each other via a passage provided with a variable orifice so that the damping forces may be adjusted in dependence on the difference in pressure between the two chambers of the different dampers.
According to this proposal, the damping forces are controlled according to the pressure difference between the chambers of two different dampers and the flow rate of fluid between the two chambers. Therefore, it is not possible to individually increase or decrease the damping forces of the two dampers that are communicated with other, or to individually control the damping forces of the two dampers both when the strokes of the two dampers are in the same phase and when they are different from each other. Therefore, depending on the dynamic state of the vehicle and road conditions, a damper force of an appropriate magnitude may not be provided, and this prevents a favorable attitude control to be achieved.